I can't control my thoughts?

This sounds really weird so I'm going to try to explain it as clearly as possible. So you know how when you're laying in bed trying to fall asleep and you decide to imagine things in your head? Well when I do this, something weird might happen in my thoughts and I can't get rid of it no matter how hard I try. For example, I remember when I was little I was thinking about how cool it would be to get up on stage and win an award. So, I pictured myself walking up on stage but then all of a sudden the person handing me the awkward had facepaint on their face and I couldn't picture them without it anymore. Or say I'm picturing myself running down a street and then all of a sudden I fall down but I can't picture myself standing back up anymore. I'll just think of something weird and then I won't be able to get it out of my head no matter how hard I try! Why do I do this, what's wrong with me? This is something different but sometimes I can't even control my thoughts at all, even during the day. They're usually very scary thoughts like someone dying and I can't get them out of my head and sometimes i'll end up crying because of them. It's stupid and weird and I don't know what's wrong with me